In Slade3, click "Edit" > "Preferences..." at the top-left, click on "Editing" at the left, and enable the "Use percent encoding if needed outside of Wad Archives" option. Yes, the name makes it sound like that won't help for WAD archives, but as far as I remember, that's what made me able to put lowercase letters and other symbols in lump names.

Keep in mind that if you try to rename multiple lumps at the same time, ^ is a special character that doesn't give the lump a name with ^ in it, so you have to rename lumps to have that in the name one by one.